1. Field of the Invention
This invention relates to shafts for moving an eddy current probe through a pipe and, specifically, to a flexible shaft with an even, protective outer sleeve supported by a data cable helically wound around a solid inner core.
2. Prior Art
It is known to have an eddy current probe for remotely obtaining nondestructive measurements of the integrity of tubes in nuclear steam generators and heat exchangers. The eddy current probe is pushed through a tube or pipe by a flexible shaft to which it is attached on the shaft lead, or distal end, the shaft extending from the probe to a data recorder with data cables running along the shaft. As the shaft impels the probe into the pipe, the probe measures the pipe along the pipe length, transmitting probe measurement data through the data cables.
Because the nuclear industry heat exchanger pipes have a small bend radius, typically less than 2 inches, it has been difficult to inspect them because of the inflexibility of conventional shaft materials. Without the capability of negotiating small-radii bends, it becomes impossible to perform a full examination of the pipes from one pipe end to another, requiring multiple passes through the pipe from different pipe access locations. Completing a full pipe measurement then results in increased inspection time, increased radiological exposure to personnel setting up the measurement equipment and increased damage to measurement equipment.
Flexible shafts designed to protect data cables and sustain compressive forces of pushing have been tried with less than satisfactory results. The shafts typically have a structure that allows them to bend within a curved pipe and provide for data cables running with the shaft. The cables are often expensive to produce and cumbersome to use. Typically, the shaft outer structure also is incompatible with a probe pusher/puller device commonly used to insert the probe in a pipe. The probe pusher comprises a reel and one or more sets of opposing rollers between which the shaft passes. The motor-driven rollers engage the shaft and rotate to urge the shaft forward, unwrapping the shaft from around the reel. As the rollers engage the shaft with sufficient force to impel the shaft the rollers often damage the shaft structure.
There exist eddy current probes that are designed to rotate within a pipe causing drive and pickup coils to rotate with the rotating probe. Shafts are designed that can rotate the probe as the shaft pushes the probe along the pipe. A spinning motor rotates the shaft as probe pusher rollers drive the shaft into a pipe. These uneven shafts are that able to simultaneously rotate and push the probe typically comprise an uneven outer surface that do not smoothly engage the pusher rollers and are often destroyed as the rollers engage the uneven shaft.
It is a first object of the invention to provide a shaft for an eddy current probe that is not damaged by a probe pusher as the shaft is driven into a pipe. It is a second object that the shaft be rotatable by a spin motor as the probe pusher drives the shaft into the pipe without damage to the shaft. It is a third object that the shaft be sufficiently flexible to negotiate tight bends in pipes yet able to sustain compressive forces without buckling as it is pushed through the pipe.